Assay tests are used in vitro medical diagnostics to detect biomolecules such as enzymes or antibodies in liquid samples derived from patients. Testing means, usually in the form of a test strip, comprise a reagent chosen to be selectively reactive with the biomolecule to be detected or a derivate thereof, yielding a reaction product, the presence of which can be seen visually or shown by detection means.
A typical prior art assay test strip is described in the publication WO 2008/018073. A non-absorbing backing laminate is arranged to support a sample pad, a reaction membrane, and an absorption pad, there being a controlled overlap between the sample pad and the reaction membrane, and between the reaction membrane and the absorption pad, respectively. A non-absorbing top laminate can be placed to partially cover said three successive parts of the strip. In use of the strip the liquid sample to be tested is supplied onto the free surface of the sample pad. The liquid phase, including the analyte, is drawn to the reaction membrane comprising one or more reagents selectively reacting with the analyte in a reaction or a series of reactions. A lateral liquid flow through the strip is generated by capillary force from the absorption pad, the latter thus accumulating liquid which has passed the reaction membrane. As materials for the strip the reference cites glass fiber or filter paper for the sample pad, nitrocellulose or a nylon membrane for the reaction membrane, and cellulose or filter paper for the absorption pad.
The usual material for the reaction membrane in commercial test strips is nitrocellulose. However, a problem with nitrocellulose is its mechanical weakness. Furthermore, a separate absorption pad of different material, usually cellulose, has to be joined with the reaction membrane to form the distal accumulator end of the test strip. In the manufacture there must be a carefully controlled overlap between the two parts glued together to ensure that the flow dynamics is uniform in each strip produced, and the absorption pad must be bulky enough to allow variable sample sizes as well as washing of the flow path in the strip or sample lines upstream of it as necessary. The use of two materials as well as the two-part structure with a glue joint between the parts thus complicate the manufacture of the strip.
The problem could be avoided by use of the same material for the reaction membrane and the absorption pad. As regards nitrocellulose the material is too brittle for that and could not be adapted to work as an absorbent. A commercial test strip, in which the membrane and the absorption pad are both of glass fibre, is available, but the lateral flow speed in such a strip is too high and causes a sensitivity problem.